This invention relates to imaging, such as medical diagnostic ultrasound imaging. In particular, new receive processing methods and systems require few transmit and receive events to form an entire image.
Commercially available medical ultrasonic imaging systems use a large number of transmit and receive events for each frame of an image. Each transmit event steers a beam of ultrasonic energy along a particular scan line and focuses this energy to a particular focus depth. After each transmit event, echoes are received, amplified and digitized. A receive beamformer generates a line of the image by dynamically focusing and apodizing the receive signals along the scan line. These transmit and receive events are repeated many times to form an image. The imaging frame rate is limited by the total number of transmit and receive events, because each transmit and receive event takes a finite amount of time determined by the speed of sound, maximum depth of interest and any system overhead processing. Using multiple beams provides a high signal-to-noise ratio and contrast resolution with a simple implementation generally immune to tissue motion.
The time limitation to transmit and receive along multiple beams is particularly acute for three-dimensional imaging. For high quality real-time three-dimensional imaging, around 30 volumes per second where each volume consists of 100 frames of data are produced. This frame rate of 3000 frames per second is not feasible given the conventional method discussed above and the speed of sound and tissue of 1500 meters per second.
One unconventional technique for increasing frame rate for scanning a region is proposed in U.S. Pat. No. 6,551,246 the disclosure of which is incorporated herein by reference. Multiple unfocused or weakly focused plane waves are sequentially transmitted at different directions. The echoes received in response to each insonnification are digitized and stored for every channel, such as system channels each connected with an element of an array. The sets of stored receive signals are delayed and apodized in multiple iterations to form component beams for each desired image point in the region insonnified by the respective waves. The final images are synthesized by adding two or more of the component beams for each image point. However, the number of calculations performed by the receive beamformer for delaying, apodizing and generating a plurality of lines of information to generate the images is high.
In another proposed approach by J-Y Lu, xe2x80x9cExperimental Study of High Frame Rate Imaging with Limited Diffraction Beamsxe2x80x9d, IEEE Trans. Ultras., Ferroelec., and Freq. Contr., vol. 45, no. 1, 1998, a normal incident plane wave is used for transmit, and multiple limited diffraction beams are formed in receive.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include a method and system for transmitting one or more plane waves into a region using a one-dimensional array to form a two-dimensional image. However, the method described here can be extended easily by those skilled in the art to three-dimensional imaging using a two-dimensional array. For signals received in response to each plane wave, a receiver applies a fast Fourier transform to generate image data. For plane waves normally incident to the transducer array, a two-dimensional Fourier transform is applied to data received for the elements or channels of the array. For plane waves transmitted at other transmit angles to the array, a Fourier transform is applied to the signals of each element independently. The resulting temporal frequency data is phased shifted as a function of the transmit angle. A Fourier transform is then applied across the elements or channels to generate spatial frequency data. The Fourier transform data is then interpolated or remapped as a function of scan angle, including the normal to the array, and any other desired variable. An inverse Fourier transform is applied to generate the image data. Either the Fourier transform data or the data generated by the inverse Fourier transform is combined for increased spatial resolution or a reduction in speckle.
Any one or more of the various aspects discussed above for receive processing allow for imaging with an increased frame rate. Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.